in search of graceful co-routines

[Chris Withers]

Hi All,

I'm looking for a graceful pattern for the situation where I have a 
provider of a sequence, the consumer of a sequence and code to moderate 
the two, and where I'd like to consumer to be able to signal to the 
provider that it hasn't succeeded in processing one element in the queue.

So, I'd want the controlling code to look a lot like:

for item in provider:
   try:
     consumer.handleItem(self)
   except:
      provider.failed(item)

Now, since the sequence is long, and comes from a file, I wanted the 
provider to be an iterator, so it occurred to me I could try and use the 
new 2-way generator communication to solve the "communicate back with 
the provider", with something like:

for item in provider:
   try:
     consumer.handleItem(self)
   except:
      provider.send('fail')
   else:
      provider.send('succeed')

..but of course, this won't work, as 'send' causes the provider 
iteration to continue and then returns a value itself. That feels weird 
and wrong to me, but I guess my use case might not be what was intended 
for the send method.

Anyway, I wonder how other people would write this?
(I'm particularly interested in a sane way to use the two way 
communication that PEP 342 introduced)

cheers,

Chris

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